1. Field of the Invention
The present invention is directed to a cardiac assist device, such as pacemaker, defibrillator or cardioverter, of the type having a unipolar atrial lead and a unipolar ventricular lead, and more particularly to an arrangement in such a device for analyzing the differential signal between the unipolar leads to identify various types of cardiac events and cardiac activity which produced the differential signal.
2. Description of the Prior Art
In the field of implanted cardiac assist devices, such as pacemakers, defibrillators and cardioverters, it is known to employ leads implanted in or around the heart in order to obtain electrical signals which are representative of cardiac events and/or cardiac activity in the heart. Various types of sensing arrangements are known for this purpose. For example, unipolar atrial sensing takes place using the tip electrode on the electrode lead which is also used for delivering pacing pulses to the atrium, with sensing taking place between the tip electrode, functioning as an active sensing electrode, and the metallic pacemaker housing, functioning as the indifferent electrode. Similarly, unipolar ventricular sensing can be accomplished using the tip electrode of an electrode cable placed in the ventricle, operated as a sensing electrode, and the metallic pacemaker housing as the indifferent electrode. Such unipolar sensing has the advantage of requiring only one electrical conductor to be contained within the particular electrode cable which is employed, thereby minimizing the diameter of the cable, and allowing a simplified cable structure, since the same electrode (the tip electrode) is used for pacing and sensing.
Another common form of cardiac activity sensing employed in implanted cardiac assist devices is bipolar sensing. For bipolar sensing, the particular lead which is used (i.e., the atrial lead, the ventricular lead, or both leads) has, in addition to the tip electrode, a ring electrode spaced a distance from the tip electrode, with sensing taking place between the tip electrode and this ring electrode. Because the signal path in bipolar sensing between the two electrodes is much shorter than in the case of unipolar sensing, the sensed signal is less cluttered with noise in comparison to a unipolar signal, since the relatively large amount of intervening tissue between the unipolar electrode and the pacemaker housing allows an opportunity for various types of noise signals to become superimposed on the actual signal produced by the cardiac event or cardiac activity. Such conventional bipolar sensing, however, has the disadvantage that it requires two electrical conductors, and associated insulation to insulate the two conductors from each other, to be contained within the implanted lead, thereby increasing the lead diameter.
A relatively recent sensing approach, differing from conventional unipolar sensing and conventional bipolar sensing, is differential sensing, sometimes also referred to as "combipolar" sensing. In this type of sensing, a unipolar lead is placed in the ventricle and a unipolar is placed in the atrium, and sensing takes place between the respective tip electrodes of these two unipolar leads. A differential signal is produced between these two leads. Sensing of this type is described in U.S. Pat. No. 5,571,143. Atrial activity is sensed between the atrial tip electrode and the ventricular electrode, while ventricular electrical activity is sensed between the tip electrode in the ventricle and the metallic housing of the pacemaker. Such a differential sensing arrangement avoids interference, such as from muscular activity, which particularly arises in the case of conventional unipolar atrial sensing, since the atrial signal is by far the weaker signal compared to the ventricular signal, but there is no need to employ a bipolar electrode in the atrium.
Since the resulting signals in differential sensing can represent activity arising in the atrium as well as activity arising in the ventricle, in order to analyze such a differential signal, there must be provided a way to identify which chamber produced the electrical activity represented in an incoming differential signal. One such approach is described in U.S. Pat. No. 5,607,457 wherein the incoming electrical signals are differentially sensed between the unipolar atrial electrode and unipolar ventricular electrode, and the sensed signal is additionally subjected to a correlation detection in order to identify which electrode is the source for the incoming signal.
In the case of conventional (i.e., non-differential) unipolar sensing, as well as in the case of conventional bipolar sensing, a large number of analysis techniques and algorithms are known for analyzing the sensed signals and identifying the type of cardiac event or cardiac activity which produced the signal for the purpose of classifying the incoming signal. Such classification, in turn, can be used for a number of different purposes, such as determining whether the signal represents normal (or desired) cardiac activity which is not in need of a change in the cardiac assist regimen, such as the pacing regimen, as well as for identifying whether some type of adjustment in the electrical therapy being administered by the cardiac assist device is needed, such as by increasing the pulse amplitude, for example, if an evoked response following an emitted pacing pulse is not detected in the sensed signal. Such classification can also be used to identify critical cardiac events which are in need of immediate attention by the cardiac assist device, such as the presence of tachycardia and/or fibrillation.
One such known technique employed in conventional unipolar sensing and conventional bipolar sensing is a pattern recognition, or waveform morphology, technique. A number of different versions of this technique are known in the art, but in general the technique involves identifying a pattern or waveform shape in all or a portion of the incoming signal representing a cardiac cycle and comparing this pattern or morphology to a number of stored patterns, each indicative of a different type of cardiac event or cardiac activity. The incoming signal is then classified according to the event or activity which is represented by the stored signal most closely resembling the pattern or morphology in the incoming signal. The pattern, in stored form, can be characterized in any of a number of different ways, such as by the occurrence of and/or the amplitude of peaks, the slope of various portions of the signal, as represented by a first differentiation of the signal, or changes in this slope, as represented by a second differentiation of the signal. Various combinations of these characteristics can also be employed to define the pattern even more precisely. Examples of these types of techniques employed in conventional unipolar and/or bipolar sensing are described in U.S. Pat. No. 4,905,708 and in the article entitled "Detection of Pathological Tachycardia by Analysis of Electrogram Morphology," Davies et al., PACE, Vol. 9, March-April 1986, pp. 200-208.